An expanded porous polytetrafluoroethylene material produced by stretching of polytetrafluoroethylene (hereinafter abbreviated as “PTFE”) has a microstructure composed of a great number of fibrils (fine fibers) and a great number of nodes connected to each other by the fibrils, and this microstructure forms a porous structure of open-cell property. In the expanded porous PTFE material, its porous structures such as a pore diameter and a porosity can be optionally preset by controlling stretching conditions.
Since the expanded porous PTFE material has the porous structure, properties such as flexibility, fluid permeability, fine particle-collecting property, filterability, low dielectric constant and low dielectric loss tangent are imparted thereto, in addition to properties such as heat resistance and chemical resistance, and surface properties such as low frictional coefficient, water repellency and non-blocking property that PTFE itself has. Since the expanded porous PTFE material has such unique properties, its applications to general industrial field and medical field, and the like enlarge. In the medical field, the expanded porous PTFE material is a material optimum for applications that directly touch intracorporeal tissues because it has properties such as chemical stability, non-toxicity to vital bodies, non-degradability and anti-thrombus property.
The PTFE itself forming the expanded porous PTFE film is a resin that is hard and brittle. On the other, the expanded porous PTFE material has good flexibility because it has the porous structure. Therefore, the expanded porous PTFE material is commonly used as cushioning materials, sealing materials and spacers. Since the expanded porous PTFE material is allowed to flexibly change its form conforming to forms of various intracorporeal tissues, or the like, it is used as medical polymeric materials such as patch materials, artificial blood vessels, catheters and artificial substitutive materials for cartilages as porous materials having a structure of a sheet or tube.
The expanded porous PTFE material is generally produced in the form of a tube, sheet (including a film), monofilament or the like. Among these forms, a sheet-like expanded porous PTFE film is commonly used for applications such as cushioning materials and sealing materials. The expanded porous PTFE film is obtained by not only forming a sheet from the beginning, but also forming a tube and longitudinally cutting the tube in the form of a sheet. It is also conducted to form tubes or various kinds of structures with the expanded porous PTFE film. For example, a tube can be formed by winding the expanded porous PTFE film on an outer peripheral surface of a rod-like support and fusion-bonding or adhesive-bonding its both ends.
However, a conventional expanded porous PTFE film is flexible, but has involved a problem that when the film is deformed by applying a load in a thickness-wise direction thereof, it is hard to recover its form to the original form even when the load is removed because the film is left great residual strain by deformation. As described above, the conventional expanded porous PTFE film is insufficient in elastic recovery property in the thickness-wise direction. Therefore, the film cannot be used repeatedly because of difficulty of recovering its form when the film is pressed in the thickness-wise direction to deform it once or at most several times. Under the circumstances, thus the expanded porous PTFE film cannot but use it only once and then discard it according to its applications.
On the other hand, repeated use is often required in applications such as a cushioning material and a sealing material. When a plurality of through-holes are provided in the expanded porous PTFE film, and a conductive metal is applied to the wall surfaces of the through-holes by a means such as plating, a flexible anisotropically conductive film can be obtained. Such an anisotropically conductive film can be used in electrical connection between circuit devices in semiconductor devices or the like or inspection of electrical reliability for circuit boards or the like. In this case, the anisotropically conductive film is pressed between terminals (electrodes) opposite to each other to conduct electrical connection or inspection of electrical reliability. However, there is room for improvement in elastic recovery property and durability upon repeated use of a expanded porous PTFE film used as a base film of the anisotropically conductive film.
Even in medical applications, the expanded porous PTFE film is used in a field of, for example, intracorporeally implanting materials such as artificial substitutive materials for cartilages. However, the expanded porous PTFE film may not exhibit its sufficient function in some cases due to insufficient cushioning property because it is hard to recover its form to the original form when it is pressed and deformed in vivo.
There has heretofore been proposed a method for causing a formed product of an expanded porous PTFE material having a form of a tube or sheet to possess rapid recovery property in its stretched direction (Japanese Patent No. 2547243). Japanese Patent No. 2547243 (hereinafter referred to as “Patent Art. 1) corresponds to U.S. Pat. Nos. 4,877,661 and 5,308,664.
Patent Art. 1 shows that when a mixture of a PTFE aggregate and a liquid lubricant is extruded in the form of a tube or sheet, and the extrudate is then stretched in its longitudinal direction, an expanded PTFE tube or sheet, which has a microstructure of nodes connected by fibrils, and in which the fibrils are stretched in the longitudinal direction, is obtained. Patent Art. 1 describes a process comprising compressing such an expanded porous PTFE material in the longitudinal direction to reduce its size, fixing the compressed state, heating the compressed expanded porous PTFE material and re-stretching it in the first stretched direction. According to this process, there can be provided a formed product of the expanded porous PTFE material, which can be stretched in the longitudinal direction and rapidly recover its length to the original length. According to the process described in Patent Art. 1, however, stretchability in a stretched direction (plane direction) can be applied to an expanded porous PTFE sheet, but elastic recovery property cannot be imparted in its thickness-wise direction.
There has also be proposed a process comprising inserting a porous polymeric material such as an expanded porous PTFE material into a mold and selectively compressing regions of the porous polymeric material in such a manner that one region has a higher density than the other region, thereby producing a compressible gasket having rigidity (Japanese Patent Application Laid-Open (KOHYO) No. 3-505596 (through PCT route)). Japanese Patent Application Laid-Open (KOHYO) No. 3-505596 (through PCT route) (hereinafter referred to as “Patent Art. 2) corresponds to WO 89/11608.
According to the process described in Patent Art. 2, however, a high-density region can be formed by selectively compressing the expanded porous PTFE material to partially enhance its rigidity, thereby retaining its shape, but elastic recovery property cannot be imparted in its thickness-wise direction.